Reductive release of DOC and iron from riparian wetland peat soils at different soil depths

Brownification of headwaters has received much attention due to implications for ecosystems, carbon storage and drinking water production. Several pathways for the mobilization of dissolved organic carbon (DOC) have been discussed in the last decades, among which recovery from acidification and increased DOC solubility at higher pH are most widely accepted. However, increased concentrations of DOC are often accompanied by a co-release of iron, raising the question of the role of reductive mobilization. To this end, we conducted an incubation experiment with samples from surficial and bottom layers (10 cm above the organic-mineral interface) of peat soil from riparian wetlands in boreal Sweden. Under anoxic conditions average release rates were 4971.4 ± 3385.5 mg m −3 d −1 for DOC and 1234.1 ± 2082.3 mg m -3 d -1 for iron. Both were significantly higher than in oxic incubations ( p < 0.001). Furthermore, pH increased and reductively released DOC was more phenolic and aromatic based on mid infrared spectroscopy. Mobilization patterns differed between the surficial and the bottom layer samples with co-release with iron being more important in the surficial layer and pH increase driving release from the bottom layer. Reductively mobilized DOC from the surficial layer had an average aromaticity index of 2.61 and from the bottom layer of 1.16. These results indicate that periods of reducing conditions in riparian wetlands and especially a connection of surficial peat soil layers to the stream during summer precipitation events lead to an increased contribution of redox induced mobilization of iron and aromatic, phenolic-rich DOC.